1. Technical Field
The present invention relates generally to memory circuits, and more particularly to an in-circuit ring oscillator and method for evaluating circuit performance.
2. Description of the Related Art
Timing of the various paths in a high-speed digital logic circuit has always been a necessity. However, with today's processor cycle frequencies reaching well above 4 GHz and with other logic circuits operating in the range of 5-10 GHz, the determination of variation and absolute delay of logic paths has become increasingly challenging.
High-speed digital circuits have been evaluated using ring oscillator circuits that duplicate a critical path with added feedback of the output to the input, providing a ring configuration. The test circuit is usually only implemented in non-production devices, or in production wafers either on the kerf, or the edge of the dies. The frequency at which the ring oscillator operates indicates the delay of the circuit. However, since such ring oscillator circuits are not the actual circuit, various factors reduce the accuracy of the ring oscillator test results. The test circuit may not completely duplicate the full circuit environment of the actual circuit, in that loading of circuits branching away from the critical path may be simulated by dummy loads rather than the actual branch paths. Further, the operating environment of the actual circuit will not be simulated in a test circuit. The environmental factors that are not duplicated include noise, thermal variation and signal coupling. For example, operation of the ring oscillator will typically result in a higher level of power dissipation and therefore higher temperature in contrast to normal operation, in which the path being simulated is not typically operating continuously.
It is therefore desirable to provide a ring oscillator circuit and method for evaluating a high-speed digital circuit design under conditions closely approximating actual operating conditions by using the actual logic circuit. It is further desirable to provide such a ring oscillator circuit and method that introduce a minimum of variation from the actual digital circuit design.